1. Field of the Invention
The present invention relates to a safety hypodermic syringe, and more particularly to such a safety hypodermic syringe, which enables the needle assembly to be received inside the barrel after its service.
2. Description of the Prior Art:
After the service of a safety hypodermic syringe, as shown in FIG. 1, the user (nurse or doctor) immediately pushes the plunger 92 forwards into engagement with the needle holder 10, and then pulls the plunger 92 backwards to receive the needle holder 10 and the needle cannula 91 inside the barrel 95.Taiwan Patent Publication #356013 discloses a similar design. However, this design of safety syringe has drawbacks. In order to let the user pull the plunger backwards with less effort after the service of the safety hypodermic syringe, the engagement power (normally friction force) between the needle holder and the inside wall of the front end of the barrel must be limited to a low level, however this design may cause the needle holder and the needle cannula to fall from position in direction toward the plunger. On the contrary, the user must employ much backward pressure to the plunger to pull the needle holder and the needle cannula back to the inside of the barrel if the engagement power between the needle holder and the inside wall of the front end of the barrel is relatively increased. Further, the production of the aforesaid safety hypodermic syringe requires high-precision fabrication equipment.
The invention has been accomplished under the circumstances in view. It is the main object of the present invention to provide a safety hypodermic syringe and a needle holder for the safety hypodermic syringe, wherein the needle holder can be shifted between an extended-out status and an inwardly squeezed status, so that the needle holder is shifted to the extended-out status and positively secured to the inside wall of the front end of the barrel of the safety hypodermic syringe during the use of the hypodermic syringe, or shifted from the extended-out status to the inwardly squeezed status for enabling g the needle holder and the needle cannula to be smoothly pulled with the plunger into the inside of the barrel after the service of the safety hypodermic syringe. According to one aspect of the present invention, the needle holder comprises a hollow cylindrical body having a rear coupling structure adapted to receive a front retaining tip of the plunger of the safety hypodermic syringe with which the needle holder is used, wherein the needle holder has at least one movable unit and at least one engagement unit respectively connected to the hollow cylindrical body, the at least one movable unit being alternatively shifted between an extended-output status where the at least one movable unit is respectively disengaged from the at least one engagement unit, and an inwardly squeezed status where the at least one movable unit is respectively forced by an external force into engagement with the at least one engagement unit. According to another aspect of the present invention, the barrel comprises an inside stop flange and an inside engagement wall portion disposed inside the inside end thereof and adapted to hold the needle holder positively in the front end of the barrel when the needle holder is shifted to the extended-out status. When pushing the plunger forwards, the at least one movable unit of the needle holder is respectively shifted from the extended-out status to the inwardly squeezed status by the inside engagement wall portion.